Methods of forming a plaster mold or core for casting metals



Patented Nov. 28, 1950 UNITED STATES PATENT OFFICE METHODS OF FORMIN G A PLASTER MOLD OR CORE FOR CASTING METALS No Drawing. Application November 4,1949,

Serial No. 125,645 V,

6 Claims- This invention relates generally to the making of molds and cores for use in metal casting and more particularly to methods of the type set forth in the Bean Patent No. 2,220,703 dated November 5, 1940.

This application is a continuation-impart of my prior copending application Serial No. 506,424, filed October 15, 1943, entitled Expansion Control of Molds and the Like, now abandoned. Reference is also made to my prior Patent No. 2,391,855.

In the manufacture of non-ferrous metal castings, particularly aluminum and aluminum alloy castings, in plaster of Paris molds made acco.ding to the process of Bean Patent 2,220,703, there is an expansion (as described in said patent) which ,in smaller castings may be an advantage, balanc- Eing out as it does at least some of the thermal ,eshrinlrage of the casting from its melting point to Eordinary temperature. One of the difficulties en- '{countered with very large industrial castings, however, is that this expansion of the mold material during its re-crystallization by the Bean process is dimcult to control exactly. Uncontrolled variations may become cumulative in the large castings and thus be very troublesome. Addition agents heretofore employed to control the processing expansion of molds (cores will be included in the scope of the term molds) have left room for improvement in various ways. Some of the best have been found to be poisonous. Others have caused objectionable efllorescence or blooming on the mold surface. Serious surface buckling has sometimes resulted. Others are good to produce a small degree of control but no way had been known to increase their effect. To overcome these difficulties requires expensive processing and constant inspection and retouching, and scrapping of dimensionally distorted molds.

A plaster molding composition composed essenof li? 312 1%? 2.52 QQQ I. and W P i. is mixed maul-5 and cast in a form or pattern whereupon, after brief delay, it sets initially to a solid condition and during further delay increases in hardness. This much of the process may be the same as in prior art processes of making plaster casts, e. g. for investment molds and for making industrial castings and art castings. According to the Bean process, however, after the initial set the mold is placed in an autoclave and subjected to steam under pressure to convert calcium sulfate (ii-hydrate to calcium sulfate hemihydrate and to leave free water distributed throughout the plaster. The mold is thereafter 5 free water from the plaster.

removed from the autoclave and allowed to stand for a number of hours at room temperature whereby the hemi-hydrate is rehydrated to dihydrate. The mold is thereafter dried to remove The efiect of this treatment is to increase the permeability of the material so as to permit escape of gases generated in the mold during casting while providing a smooth skin at the surface of the mold giving 10 faithful reproduction in the surface of the cast- The problem of controlling expansion during such processing, by use of chemical agents is complicated and requires that one take into consideration not only the efiects, if any, on both the initial setting expansion and the subsequent processing expansion and room temperature rehydrating of the Bean process, but also the effect of any control-agent on other ma,or properties and attributes of the mold material such as setting time, green strength, processing time, surface quality, dry strength, permeability and heat absorption rate. For example, some expansign control agents are powerful accelerators, ba'ifsing -the plaster to set almost" instantly. Others are powerful retarders or inhibitors of setting. Some agents will cause the green strength to be but a fraction of what it would be otherwise. Some have the adverse effect of reducing permeability of the finished mold. All

these may seriously afiect the economics of the process.

I have now discovered that a water soluble e1 kali silicate particularly sodium silicate an /or 5 p*o' assium silicate, added 'in small amount controls both the total expansion of the plaster and particularly that due to the dehydrating and subsequent rehydrating steps of the Bean process; and that these silicates otherwise successfully 0 meet the requirements of a suitable expansion control agent as outlined. Surprisingly also a lesser amount of such a silicate can be used with an expansion control agent which is satisfactory except for producing too little control, such as 45 Portland cement, for example; and the combi- 55 is required. Very satisfactory results have been 3 obtained with an addition of about 1% by weight of the sodium or potassigm si ligate. Best results have been obtained with the addition of .4% to .8% by weight of the least alkaline commercial ow ere .s iu. silicate (Philadelphia Quartz G Brand). In general he amount of the solu e si ica should be from 0.1 to 1.0%, the

highlyjemjveTand'rconQngt 'al The Bortland cement decreases somewhat the expansion during mold processing but if more than a small amount is used it adversely affects the permeability of the mold when treated by the Bean process. 0.5% is the maximum which can be recommended and more than that gives very little improvement in dimensional control. Ordinarily no more than 0.4% should be used.

Portland cement has the advantage that it does not migrate to the surface; and by using 0.4% of Portland cement plus 0.3% of sodium silicate (Phi adelphia Quartz E Brand) one can get excellent dimensional control in the largest casting molds.

The G brand silicate referred to above is one having an NazO1SiO2 ratio 1:3.22. Other grades having alkalinity up to a 1:2 ratio may be similarly used.

The dry mix as thus prepared may then have water added thereto to'form the wet mix from which the mold is formed. After the wet plaster forming the mold has set, the mold is processed with st m under pressure in an autoclave to convert fi i ecalbimnfiffitedi-hydrate to calcium sulfate hemi-hydrate and to leave free water distributed throughout the mold. The mold is then removed from the autoclave and allowed to stand at room temperature for a number of hours whereby the hemi-hydrate is rehydrated to dihydrate. The mold is then dried at a temperature between 200 F. and 475 F. The temperature should not exceed 500 F. as that generally results in shrinkage, cracking and distortion.

An actual comparative test as follows indicates the advantage gained by the present invention: A mold composition was employed consisting of 61% plaster of Paris, 21% silica flour, 17% talc and 1% asbestos. This composition was processed in accordance with the teachings of Bean Patent 2,220,703 and wherein the plaster as cast was allowed to set for fifteen minutes; the plaster casting or mold was dehydrated in an autoclave operated at 17 pounds gauge pressure for nine hours; re-hydrated under room conditions for 14 hours; and dried in a core oven at 350 F. for 12 hours. The resulting control or standard mold made from this composition, and without addition of any expansion control agent, showed an average expansion of .0093" per linear inch in its over-all dimensions when in final processed condition.

An identical procedure with the same composition except for addition of 1% sodium silicate (based on the dry weight) produced a casting or mold wherein, after the same treatment, the expansion was .0018" per linear inch. Addition of 1.00% of sodium silicate (QQ Brand) to the said composition produced a cas mg or mold Thus a combination of Portland cement and sodium silicate has proven wherein the expansion was only .0021" per linear inch. Additions of .8% of potassium silicate to the said control composition insFad of the sodium silicate, gave after said treatment the 5 same total expansion, .0021" per linear inch.

In another demonstration series for the controlling effect of sodium silicate and potassium silicate, another mold material composition was used consisting of 42% glgtsgmjgisixflfi z, sand (round grain O awa 50 mes 7% ac 2.4% sili flour and 1% w This was mixed W1 "water in the usual way and cast into a control or standard mold block containing a rectangular cavity deep, 2" wide and 10" long. In one procedure no expansion control agent was employed, the conditions of mold i6?- mation and treatment were the same as those described above in the preceding tests. This mold block or casting when no expansion control agent was employed and Which was treated as above indicated showed an expansion from its original cast or green condition of between .009" and .010" per linear inch.

For comparison with the above, other molds were made from a mixture including 1.00% by weight of sodium silicate (CG Brand) and otherwise identmfioldinateiial of the control just described. This composition was formed into a mold and treated in the same manner as that employed in the control." After being subjected to the dehydration, rehydration and drying conditions above specified, this mold block showed an expansion of only .0007" per m linear inch over its dimensions when in green condition.

In a third procedure 0.75% sgdiumsilicateiCG Brand} was added to the mixture of rafiadfi rol and made in the same manner. Its expansion was .001" per linear inch. In a fourth com- 40 parison 0.5% of sodium silicate (CG Brand) was added to the mixture of the control giving, when processed in the same manner, a mold having an expansion of .0026" per linear inch.

As an example of the invention using ua agents for ex ansion control to minimize t e en ency 1n largeim'oldst6 migration of the soluble silicate toward the surface the mold material composition consisted of 42% plaster of g p 45.4% sand, 2.4% silica fidfiffatwa,

ftfgr efuz, g foa z, gortfifid ifierit. 0.5% erral alba nely ground raw gypsum) an 0.3% Wilicate (Philade p 1a" uartz 11g 'i' nixture was mixed with water and poured and processed in accordance'with the teachings of said Bean patent under the conditions above stated and produced a mold block wherein the expansion was .0019" per linear inch. A mold block made from the same composition but with the sodium silicate omitted and under the same processing conditions as those employed with the sodium silicate included produced a mold block wherein the average expansion was .0055" per linear inch.

It appears from the results of the foregoing examples and demonstrations that effective control over expansion of plaster molds made and treated under the conditions set forth in said Bean patent is achieved by the use of the special expansion control agents set forth namely the soluble alkali silicates. It is also contemplated that insoluble expansion control agents e. g. Portland cement, which are not sufiiciently efiective when used alone, may be advantageously combined in the same mold composition with a soluble expansion control agent, e. g. KCl, which by CROSS 5 itself cannot be tolerated in the quantities required if it is to perform the expansion control function alone. (KCl, in quantities sufiicient to control expansion within practical limits, will cause serious surface deterioration because of its tendency to migrate through the mold.)

In one application of the invention in which plaster molds for small aluminum alloy castings were made the average permeability of the molds was in the range of 50 to 300, as compared to a range of 10 to 30 when the silicate was not used as an expansion control agent but with the composition and conditions otherwise identical. The actual permeability values in each range depend on the amount of water used in wet mixing and on rehydrating conditions. Efllorescence and blooming were eliminated almost entirely.

While the specific disclosures of the invention described herein deal with the formation of plaster molds for casting metal, it will be understood that the principles of the invention have application in the formation of other plaster bodies in which expansion is required to be closely con- .trolled.

I claim:

1. The method of forming a plaster mold which includes forming a desired shape from a mixture comprising plaster of Paris, an inert filler, a water-soluble alkali metal silicate selected from the class consisting of sodium silicate and potassium silicate as the essential expansion control agent in an amount between 0.1% and 1.0% of the dry weight of the plaster composition, and water; setting the plaster; dehydrating the set plaster to convert calcium sulfate (ii-hydrate to calcium sulfate hemi-hydrate and to leave free water distributed throughout the mold; thereafter rehydrating the hemi-hydrate to the dihydrate; and then drying the mold to remove practically all of the remaining free water from the plaster, the temperature to which the plaster is subjected throughout the entire plaster conditioning treatment and prior to casting molten metal into the mold not exceeding 500 F.

2. The method as in claim 1 in which about .8% by weight of sodium silicate is employed.

3. A method of making plaster molds and the like for use in casting metals wherein the extent of the processing expansion which occurs in transition of said mold from the wet state on a pattern to a dry state after processing as hereinafter specified, is confined within relatively small limits, said method including forming on a pattern of predetermined dimensions a plaster mold composition comprising plaster of Paris, an inert filler, an expansion control agent including a water-soluble alkali metal silicate selected from the class consisting of sodium silicate and potassium silicate in an amount between 0.1% and 1.0% of the dry weight of the plaster composition, and water; and after initial set, heating said mold to a temperature above that at which water is released from calcium sulfate di-hydrate and under super atmospheric pressure at which the released water is held in liquid state, rehydrating the calcium sulfate by cooling the mold, and then drying said mold to remove free water therefrom at temperatures below 475 F., the temperature to which the plaster is subjected throughout the entire plaster conditioning treatment and prior to casting molten metal into the mold not exceeding 500 F.

4. The method as defined in claim 3 wherein the alkali metal silicate is a sodium silicate with a NazozsiOz ratio of from 123.22 to 1 :2.

5. The method as defined in claim 3 wherein the expansion control agent includes a significant amount but not over 0.5% of Portland cement and a significant amount but not over 0.5% of the water-soluble alkali metal silicate.

6. The method of controlling expansion due to changes of calcium sulfate in the plaster between its di-hydrate and hemi-hydrate forms, in the making of plaster molds and the like by a process including the forming of a plaster molding composition on a pattern of predetermined dimensions, dehydrating calcium sulfate di-hydrate in said composition to calcium sulfate hemi-hydrate and rehydrating to the (ii-hydrate, the temperature of the plaster not exceeding 500 F. throughout the entire plaster conditioning treatment and prior to casting molten metal into the mold, which method is characterized by incorporating in the plaster molding composition a water-soluble alkali metal silicate selected from the class consisting of sodium silicate and potassium silicate in an amount between 0.1% and 1.0% of the dry weight of the plaster composition and an insoluble expansion control agent in substantial amount but less than about 0.5%.

XARIFA L. BEAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,078,200 King Apr. 20, 1937 2,220,703 Bean Nov. 5, 1940 2,263,586 Neiman Nov. 25, 1941 2,391,855 Bean Dec. 25, 1945 

1. THE METHOD OF FORMING A PLASTER MOLD WHICH INCLUDES FORMING A DESIRED SHAPE FROM A MIXTURE COMPRISING PLASTER OF PARIS, AN INERT FILLER, A WATER-SOLUBLE ALKALI METAL SILICATE SELECTED FROM THE CLASS CONSISTING OF SODIUM SILICATE AND POTASSIUM SILICATE AS THE ESSENTIAL EXPANSION CONTROL AGENT IN AN AMOUNT BETWEEN 0.1% AND 1.0% OF THE DRY WEIGHT OF THE PLASTER; DEHYDRATING THE SET PLASTER TO CONVERT CALCIUM SULFATE DI-HYDRATE TO CALCIUM SULFATE HEMI-HYDRATE AND TO LEAVE FREE WATER DISTRIBUTED THROUGHOUT THE MOLD; THEREAFTER REHYDRATING THE HEMI-HYDRATE TO THE DIHYDRATE; AND THEN DRYING THE MOLD TO REMOVE PRACTICALLY ALL OF THE REMAINING FREE WATER FROM THE PLASTER, THE TEMPERATURE TO WHICH THE PLASTER IS SUBJECTED THROUGHOUT THE ENTIRE PLASTER CONDITIONING TREATMENT AND PRIOR TO CASTING MOLTEN METAL INTO THE MOLD NOT EXCEEDING 500*F. 